The present invention relates generally to improvements of multi-phase brushless motors, each of which is applicable particularly, but not exclusively, to a drive system of a capstan and so on of a video tape recorder.
Recently, brushless motors are being increasingly miniaturized in order to meet the requirement of size-reduction for video tape recorders, for example. The miniaturization of the brushless motor causes decrease in the inertia force of a rotor thereof which in turn results in occurrance of considerable irregularity of rotation due to torque ripple. One current technique for resolution of this problem involves constructing a brushless motor with multi-phase drive coils to lower the torque ripple, the torque ripple decreasing with increase in the phase number as shown in FIG. 1. However, an extreme increase in the phase number causes a complex drive circuit arrangement and hence increase in production cost of the brushless motor. More specifically, Hall generators are provided in the brushless motor in order to detect the rotational position of the rotor and the number of the Hall generators depends upon the number of phases. Furthermore, the increase in the number of the Hall generators results in increase in the number of lead wires and so on, connected thereto, so as to make difficult the size-reduction and assembly of the brushless motor.
Thus, a brushless motor has been recently developed such as disclosed in Japanese Patent Provisional Publication No. 61-66586 and illustrated in FIG. 2, the arrangement of which is made so as to effect the supply of drive currents to drive coils on the basis of FG signals. In FIG. 2, the brushless motor includes a detection signal processing circuit 12 which is coupled to a magnetic sensor 6 to count speed-detection pulses from the magnetic sensor 6. The detection signal processing circuit 12 successively supplies exciting signals to transistors TR1 to TR10 to supply a drive current to phase coils U to Y. The magnetic sensor 6 is also coupled to a speed control circuit 11 which in turn determines the drive current in correspondance with the speed of a rotor 1. There is a problem which arises with this type of brushless motor, however, in that its operation becomes unstable because difficulty is encountered to sufficiently obtain the FG signal at startup and low-speed rotation, and particularly the self-starting becomes impossible in the case of a large load being applied to an output shaft.